Distribution and ecology of benthonic foraminifera in the sediments of the visakhapatnam shelf, east coast of India

Abstract

Of a total of 124 benthonic foraminiferal species identified from the sediments of the Visakhapatnam shelf, east coast of India, only 17 species are abundant. Bolivinia vadescens is cosmopolitan and abundant in the entire shelf. Ammonia beccarii, A. papillosa, Asterorotalia trispinosa, Bolivina compacta, Cibicides lobatulus, Discorbis australis, Florilus grateloupi, and Hanzawaia concentrica are abundant in the inner shelf; Amphistegina radiata, Heterolepa dutemplei, and Uvigerina proboscidea in the middle shelf; while Bolivina robusta, Bulimina gibba, Cassidulina laevigata, Uvigerina bifurcata, and U. peregrina occur in the outer shelf. The benthonic population sizes are larger in the outer shelf than in the inner shelf, but species diversity is higher in the middle shelf than elsewhere. The species are arranged in 10 bathymetric groups which show that 20 species are restricted to the outer shelf, 29 to the inner shelf, and 10 to the middle shelf while 24 species occur over the entire shelf. The foraminifera are grouped into dominant, characteristic, and accessory species and these groups allow recognition of four biofacies in the Visakhapatnam shelf, restricted respectively to the depth zones of 0–20 m, 20–55 m, 55–90 to 105 m and 90 to 105–190 m. Bottom water temperature and currents, rather than sediment attributes and water salinity, influence the distribution patterns of the species and their population densities in sediments of the Visakhapatnam shelf.